Generally, there is a demand for a tire excellent in on-ice performance, in particular, on-ice braking performance, as a pneumatic tire suited for use on an icy road.
Here, the braking performance and the traction performance of a tire are usually subject to influence of the friction properties of the tire. Therefore, in order to improve the on-ice braking performance of the tire, the on-ice friction properties of the tire need to be improved.
There are known various methods to improve the on-ice friction properties of a tire. Such methods include ensuring the ground-contact area of the tire, and improving the scratch effect to be produced by the edges of blocks and the edges of sipes formed on the tread portion against an icy road.
However, in a conventional pneumatic tire having a plurality of main grooves extending along the tire circumferential direction and a plurality of lateral grooves extending along the tire width direction formed on a tread portion surface so that rectangular blocks are partition-formed, and having sipes formed in the blocks (see, for example, JP H07-186633 A), when the blocks 70 are increased in rigidity so that the blocks 70 can be prevented from being inclined to thereby ensure the ground-contact area of the tire as can be seen from FIG. 13(a) schematically illustrating a state of the blocks during running, the scratch effect to be produced by the edges against an icy road G cannot be fully obtained. Meanwhile, in the conventional pneumatic tire, as can be seen from FIG. 13(b) schematically illustrating another state of the blocks during running, when the inclination of the blocks 70 is enhanced for the purpose of obtaining the scratch effect of the edges against the icy road G, the kick-out end side of each of the blocks 70 comes off the road surface, failing to ensure the ground-contact area.